The present invention relates to the field of electro-acoustic transducer circuits. More specifically, the present invention relates to the inductive tuning of capacitive electrostatic micro-fabricated electro-acoustic transducers.
An electro-acoustic transducer is an electronic device used to emit and receive sound waves. These transducers are used in medical imaging, non-destructive evaluation and other applications. Ultrasonic transducers are electro-acoustic transducers that operate at higher frequencies, typically at frequencies exceeding 20 kHz.
The most commonly used type of ultrasonic transducer is the piezoelectric transducer (PZT) made of ceramic materials. In recent years, a revolutionary, new technology has been developed with the potential of displacing conventional piezoelectric ceramic-based ultrasound transducers used for medical ultrasound imaging. These new transducers are made of fine micro-fabricated membranes suspended above Silicon-based substrates. These transducers operate in an electrostatic mode and electrically approximate a parallel-plate capacitor with finely spaced plates. These micro-fabricated transducers have considerable potential since the micro-fabrication process gives rise to low cost, highly complex structuresxe2x80x94such as finely pitched 2D arrays of elements. Furthermore, since the micro-fabricated transducers are based on Silicon, it is envisioned that suitable driver and receiver circuitry may be integrated onto the same Silicon substrate or onto one immediately adjacent to the transducer substrate. Thus, the micro-fabrication technology may enable 2D arrays and real-time 3D imaging, which until now has been hampered by the cost and complexity of the cumbersome, time consuming, low-yield manufacturing processes required for the ceramic-based arrays. The micro-fabrication technology may also enable new intravascular applications such as placing transducer arrays on the tips of catheters or on other temporary, or semi-permanent, minimally invasive monitoring instrumentation used inside the body to monitor physiological functions (e.g., blood flow, blood pressure, etc.).
One drawback of the electrostatic micro-fabricated transducer arrays is that they substantially behave with the electrical characteristics of a capacitor. The capacitance of the micro-fabricated transducer introduces a negative reactance component to the overall transducer impedance, which makes the transducer inefficient. What is needed is a way to tune out the negative reactance of the micro-fabricated transducer using inductive tuning, thereby making the transducer circuit more efficient. However, inductive tuning alone results in narrowband operation, which is also undesirable, because the narrowband operation prevents the transducer circuit from performing efficiently for harmonic imaging, which requires a broader operating bandwidth.
Harmonic imaging (i.e., filtering receive signal to around the second harmonic of the transmitted signal) has recently become the default imaging mode in medical diagnostic ultrasound. It has been found that by imaging the nonlinearly generated harmonic signal, one gets a far superior image in terms of both spatial and contrast resolution. Harmonic imaging applies to both imaging of tissue alone or imaging of introduced contrast agents. Harmonic imaging requires a moderate to high sound intensity since it is based on a nonlinear effect. Additionally, harmonic imaging inherently requires high transducer bandwidth or, alternatively, the ability to switch the frequency of high sensitivity between transmit and receive. Ultimately what is needed, is a solution to the problem for operating an inductively tuned, capacitance-based micro-fabricated transducer efficiently for harmonic imaging.
It is an advantage of the present invention to provide a method for tuning out the negative reactance of a capacitive micro-fabricated electrostatic transducer, such as by using inductive tuning, thereby making the transducer circuit more efficient.
It is a further advantage of the present invention to provide a method for inductively tuning a capacitive micro-fabricated electrostatic transducer efficiently for harmonic imaging.
Still further, it is an advantage of the present invention to provide a capacitive micro-fabricated electrostatic transducer circuit with the negative reactance tuned out using inductive tuning, thereby making the transducer circuit more efficient.
It is a further advantage of the present invention to provide a capacitive micro-fabricated electrostatic transducer circuit, inductively tuned for efficient use in harmonic imaging.
The present invention achieves the above advantages, among others, singly or in combination, by providing an electrostatic transducer circuit in which a balancing inductance is inserted into an electrostatic transducer circuit. The electrostatic transducer circuit generally includes transmit circuitry, receive circuitry and a capacitive electrostatic transducer. The balancing inductance is tuned to counteract the negative reactance of the capacitive electrostatic transducer at a desired operating frequency during the transmit mode. The balancing inductance is inserted into the transmit circuitry and is then isolated from the remaining parts of the electrostatic transducer circuit. Isolation is achieved by switching the electrostatic transducer circuit between transmit and receive modes of operation. Further, a receive circuit balancing reactance can also be included.
In addition, the present invention achieves the above advantages, among others, singly or in combination, by providing a method of for tuning out the negative reactance of a capacitive micro-fabricated electrostatic transducer. The method provides a balancing inductance that is used to counteract negative reactance of the capacitive electrostatic transducer at a desired operating frequency during transmit mode.